I really like the Rime of the Ancient Mariner by Coleridge. I first encountered it as a result of reading Douglas Adams' Dirk Gently novels, but one day I saw the original in the library and just read it from start to finish. It's fantastic, so weird, so compelling.
I also like his Kubla Khan, the imagery of the "caverns measureless to man" and the "sunless sea" have always stuck with me.